Lethal shockwave from an island in the sun

Volcanic activity on the Canary Isles could send a tidal wave to devastate
Florida. Phillip Henry monitors the changing shape of La Paima

The Independent - Monday 24th June 1996

It reads like the plot from a disaster
movie. Florida is devastated by a
tidal wave tens of
metres high. The destruction and
loss of life is immeasurable.
The wave which caused so
much devastation crossed the
Atlantic in just a few hours, unseen
until it reached the American coast.
Its source is an unstable geological
fault on the Canary Isles, more
usually thought of as an idyllic holiday
destination of thousands of
European tourists than as the cause
of disaster.

To ensure such a scenario remains
in the realms of Hollywood, a group
of British scientists recently travelled
to the Canaries. By monitoring the
fault which threatens to create
the tidal wave, they hope to
predict any hazard long before it
could happen.

In the middle of the ocean, these
waves of mass destruction - called
Tsunami - are almost invisible. Only
when they reach the shallow waters
around coasts do they become huge
breakers.

The ruin caused by even a relatively
small one can be apocalyptic.
When the Krakatoa volcano blew itself to pieces in the last century, a
tsunami six metres high killed 30,000 people.

Tsunami can also be generated
when a huge landslide falls into the
sea. This has never been seen in
historical times but scientists have now
identified the island of La Palma as
a potential hazard.

"There is a danger that the side
of the volcano facing west may fall
into the Atlantic," says Professor Bill
McGuire of the Centre of Volcanic
Research in Cheltenham, who was
part of the recent expedition.

"It could literally happen during
the next few weeks or months or
years," he said. "Equally, it could
happen 100 years or more into the
future. The island is very unstable
and this is something which could
happen fairly soon."

La Palma is not only the steepest
island in the world but has also been
the most volcanically active of the
Canary Isles in the past 500 years.
There have been two eruptions on
the island this century alone - the
last one was in 1971.

The volcanoes themselves do. not
present much danger. La Palma
lava moves so slowly that most people
could easily outrun it, so there
is no cause for anxiety to the many
tourists who visit the island. The real
danger lies in the possibility that an
eruption might trigger the collapse
of a volcanic ridge which is unsound.

The problems started when an
eruption in 1949 caused several
cubic kilometres of rock to slide a
few metres toward the sea. This also
opened a two-kilometre-long fracture
which can quite easily be seen
to this day.

There are not only fears that a
future eruption would cause the
rock to move again, but that next
time, the landslide will not stop. If
this happened, the resulting tsunami
would be catastrophic.

"There have been three of these
collapses in the history of the
island," says Juan Carlos Carracedo
of the Spanish National Research
Council. Not only does the landscape
bear the scars of these cataclysms
but submarine photos show
rock from the peaks of old volcanoes
far out to sea. "Another collapse is
impending. The only way to prevent
this hazard is to study the island
closely."

By monitoring the change in
shape of the mountainside, the team
hope not only to discover if the western
flank is slipping due to gravity
but to predict if the sleeping volcano
is growing restless. Before eruptions,
volcanoes always swell. This swelling
may be imperceptible to the human
eye. Only by surveying the shape of
the ground with sensitive instruments
can small changes be
detected.

The team of scientists used a system
called electronic distance measuring
(EDM). By bouncing an
infra-red beam off a mirror on
another ridge of the volcano and
timing how long the beam takes to
return, the EDM can be used to
measure distances to an accuracy of
a few centimetres per mile.

In late 1994, scientists set up a
network of stations on the mountainside
and accurately measured
the distances between them. After
one and a half years, they returned
to measure the network again. If the
distances between the stations had
become greater over that period.
this would suggest that either the
fault had slipped or the ground was
bulging as molten rock inflated the
volcano.

For the moment, results show
there has been no movement. While
the rest of us might breathe a sigh
of relief, the measurements are
highly valuable to the scientists
because they give them a "baseline",
illustrating the behaviour of
the volcano under normal
conditions.

Should future studies reveal that
the volcano has deviated from this,
the prospect of the cast coast of
America being flooded by a wave
from the other side of the Atlantic
may turn from fiction to horrifying
reality.